1. Field of the Invention
The present invention relates generally to the field of solid-state image pickup devices such as a CMOS image sensors. More specifically, the present invention is directed to a solid-state image pickup device using a so-called column system and a method of controlling the device such that a pixel signal obtained by a photoelectric converting region unit is sequentially stored in a column region unit provided for each pixel column and the column region unit is sequentially selected to sequentially output each pixel signal.
2. Description of the Related Art
FIG. 12 is a circuit diagram showing an example of configuration of a CMOS image sensor using a column system. The CMOS image sensor includes, on a semiconductor substrate (not shown): a photoelectric converting region unit 1 formed by a two-dimensional pixel array; a horizontal scanning circuit 9 for making a selection in the horizontal direction; a vertical scanning circuit 10 for making a selection in the vertical direction; a timing generator unit 11 for generating various timing signals; an output amplifier 12 for amplifying a pixel signal; a PGA (programmable gain control amplifier) circuit 13; an AD (analog-to-digital conversion) circuit 14; and the like.
The CMOS image sensor uses a column system in which a capacitor 6 is provided for each pixel column in a part referred to as a column region provided on an output side of the photoelectric converting region unit 1, and a signal read from each pixel is sequentially stored in the capacitor and sequentially output to the output amplifier 12. Since signal processing on each pixel signal is performed after reading the pixel signal in a unit of a pixel column, a configuration within each unit pixel is simplified as compared with a device that performs similar signal processing within each unit pixel, so that provision can be made for an increase in the number of pixels, reduction in size, reduction in cost and the like of the image sensor.
Operation of such a circuit will next be described briefly. The photoelectric converting region unit 1 for receiving light signals includes a plurality of unit pixels P (1-1-1, 1-1-2, 1-1-3, . . . ) arranged in a row and a column direction. Such a unit pixel includes therein at least one photoelectric converting device. As the photoelectric converting device, a photodiode or a photogate is generally used.
For pixel signals to be output from the photoelectric converting region unit 1, the vertical scanning circuit 10 selects a predetermined row in order via a control line 3 (3-1, 3-2, 3-3, . . . ).
Incidentally, while only one control line 3 is shown in each pixel row in FIG. 12, a plurality of control lines 3 are generally provided in parallel with each other in each pixel row for the vertical scanning circuit 10 so that it may select each pixel row in order to read pixel signals.
Then the signals of the row selected via the control line 3 are sequentially stored in capacitors 6 (6-1, 6-2, 6-3, . . . ) of column region units disposed in parallel on the output side of the photoelectric converting region unit 1. The operation of storing the signals from the row is performed simultaneously.
The pixel signals stored in the capacitors 6 of the column region units are sequentially selected by operation of the horizontal scanning circuit 9 scanning columns from a leftmost one in order. That is, the horizontal scanning circuit 9 selects and drives column selecting transistors 7 (7-1, 7-2, 7-3, . . . ) in order. Thereby the pixel signal of each of the pixels P is read in order.
The output amplifier 12 sequentially amplifies the pixel signal output to a horizontal signal line 8 and then outputs the result as a voltage signal. The PGA circuit 13 amplifies voltage of the voltage signal in small gain steps. The voltage-amplified pixel signal is input to the AD circuit 14, and then output as a digital signal 15 to the outside of the semiconductor chip.
Incidentally, a vertical signal line is supplied with a fixed bias voltage by a bias circuit 2 via load transistors 5 (5-1, 5-2, 5-3, . . . ). The AD circuit 14 now developed and used in the conventional solid-state image pickup device described above generally has a bit precision of 12 bits or 14 bits. When the number of bits of the AD circuit 14 is increased, power consumption is increased and noise of the circuit itself makes it particularly difficult to improve the bit precision.
Thus the conventional image sensor using the column system has problems in that it is difficult to improve the bit precision and it is not possible to extend the dynamic range while maintaining the S/N ratio at a favorable level.
Incidentally, when the gain of a pixel signal is controlled on a pixel-by-pixel basis within each pixel of the photoelectric converting region unit 1, the photoelectric converting region unit can improve the bit precision and extend the dynamic range of an output signal. In this case, however, the configuration of each pixel becomes complicated, and hence advantages of reduction in cost and reduction in size provided by the above-described column system cannot be obtained.
It is accordingly an object of the present invention to provide a solid-state image pickup device and a control method thereof that make it possible to control the gain of the pixel signal on a pixel-by-pixel basis and extend the dynamic range while maintaining the S/N ratio at a favorable level even in a circuit configuration using the column system. Other objects and advantages of the present invention will be apparent in light of the following Summary and Detailed Description of the presently preferred embodiments.